Page:Proceedings of the Royal Society of London Vol 60.djvu/447

Rh to tbe muscles of antagonistic position acting about certain joints of the limbs.

If transection of the neural axis be carried out at the level of the crura cerebri in, e.g., the cat, there usually ensues after a somewhat variable interval of tim e a tonic rigidity in certain groups of skeletal muscles, especially in those of the dorsal aspect of the neck and tail and of the extensor surfaces of the limbs. The details of this condition, although of some interest, it is unnecessary to describe here and now, except in so far as the extensors of the elbow and the knee are concerned. These latter affect the present subject. The extensors of the elbow and the knee are generally in strong contraction, but altogether without tremor and with no marked relaxations or exacerbations. On taking hold of the limbs and attempting to forcibly flex the elbow or knee a very considerable degree indeed of resistance is experienced, the triceps brachii and quadriceps extensor cruris become, under the stretch which the more or less effectual flexion puts upon them, still tenser than before, and on releasing the limb the joints spring back forthwith to their previous attitude of full extension. Despite, however, this powerful extensor rigidity, flexion of the elbow may be at once obtained w ith perfect facility by simply stimulating the toes or pad of the fore foot. W hen this is done the triceps enters into relaxation and the biceps passes into contraction. If, when the reflex is evolved, the condition of the triceps muscle is carefully examined, its contraction is found to undergo inhibition, and its tenseness to be broken down synchronously with and indeed very often accurately at the very moment of onset of reflex contraction in the opponent prehrachial muscles. The guidance of the flexion movement of the forearm may therefore be likened to that used in driving a pair of horses under harness. The reaction can be initiated in more ways than one, electrical excitation of a digital nerve or mechanical excitation of the sensory root of any of the upper cervical nerves may be employed; I have seen on one occasion a rubbing of the skin of the cheek of the same side effective.

Similarly in the case of the hind limb. The extensor muscles of the knee exhibit strong steady non-trem ulent contraction under the appropriate conditions of experiment. Passive flexion of the knee can only be performed with use of very considerable force, the quadriceps becoming tight as a stretched string. The application of hot water to the hind foot then elicits, nevertheless, an immediate flexion at knee and hip, during which not only are the flexors of those joints thrown into contraction, but the extensors of the knee joint are simultaneously relaxed. Electric excitation of a digital nerve or of the internal saphenous nerve anywhere along its course will also initiate the reflex.

The same relaxation of existing contraction in the extensors can